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ETFE Membrane Structures: and What About Hail Impact Resistance?

May 15, 2007

Doorways to the Future
RCI 22nd International
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Proceedings of the RCI 22nd International Convention Flüeler – 29
ETFE Membrane Structures: and What
About Hail Impact Resistance?
Peter H. Flüeler
Swiss Federal Laboratories for Materials Testing and Research
Flüeler Polymer Consulting GmbH
Zurich, Switzerland
Doorways to the Future
Ethylene-tetra-fluoro-ethylene (ETFE) membrane structures have become very fashionable
in Europe since large, light-transparent buildings like the Eden Project in
Cornwall, the Space Centre in Leicester, England, and the Masoala Rainforest
Building (MRF) in Zurich and others became very popular. In Europe, increasing hail
intensity and frequency might be the result of climate changes. Alpine countries such
as France, Switzerland, Germany, Italy, and Austria are increasingly hit by hailstorms.
Therefore, buildings must exhibit increased resistance against hail impact.
Current test procedures often require a hailstone size of 40 mm (approximately 1.6-
inches) and higher. Experiences during the recent hailstorms will be explained with
reference to the MRF building, where, after the first hailstorm in 2002, a protection
system was mounted. In another hailstorm occurring two years later, the installed
system proved its efficiency.
PETER H. FL ELER has 26 years experience in polymer technology of building materials
and their long-term performance. He has done extensive hail resistance and
polymer material testing and evaluation. Flüeler has lectured to architects and engineers
at the Swiss Federal Institute of Technology ETH, Zurich.
Flüeler – 30 Proceedings of the RCI 22nd International Convention
In the last ten years, many
large sculptural buildings with
special optical effects were
designed and built for park decks,
gardens, museums, exhibitions,
sports facilities, etc. Spectacular
examples in which ethylene-tetrafluoro-
ethylene (ETFE) was used
for roof and wall coverings are the
Eden Project in Cornwall, Great
Britain; the Air and Space Center,
Leicester, GB; the Masoala
Rainforest MFR building in
Zurich, Switzerland; and Allianz
Arena, Munich, Germany. More
such buildings are under study or
in the stage of realization in China
and in other countries.
Important requirements of
such buildings are high
transparency, light
weight, visual effects,
sculptural tolerance, or
possibilities of unconventional
shape, long
service life, and low cost.
It seems that ETFE
membranes combined
with steel framework
constructions do meet
most of these requirements.
In this article, the
experiences of the MRF
building at the Zurich
Zoo during two hailstorms
in 2002 and
2004 will be discussed.
The study and realization
of a protection
against hail impact will
be presented.
Material Properties
ETFE is the abbreviation of a
semi-crystalline thermoplastic
polymer usually composed of
polyethylene (PE) and tetra-fluoro-
ethylene (TFE) in a ratio of 1 to
3. By inserting P-segments into
the stiff TFE molecular chain,
good melting properties are
achieved, facilitating the extrusion
process. Hence, thin foils can
be manufactured with a slightly
bluish color if not filled with pigments.
Such foils are light- and
UV-transparent. They have a high
tear resistance, high melting temperature,
excellent surface properties,
good aging behavior, outstanding
resistance against chemicals,
and good fire properties.
They can be colored and the surface
treated for special appearance.
The main properties are listed
in Table 1.
Drawbacks are point and
shock loadings because of their
thinness. Special attention is
required to mechanical influences
from installation and maintenance.
Vulnerability to vandalism
and sound transmission of such
structures must be respected during
design, construction, and use.
At present, the membranes can be
welded by hot air or high frequency
techniques only under controlled
factory conditions. Repairs
Proceedings of the RCI 22nd International Convention Flüeler – 31
ETFE Membrane Structures: and What
About Hail Impact Resistance?
General properties
Density 1.73 g/cm3
Mass of 0.1 mm membrane 0.173 kg/m2
Appearance, non-pigmented glossy, transparent, slightly bluish
Surface smooth, water repellent
Main mechanical properties (0.2mm thick)
Tension: E-modulus (long./perp.) 1060 /1069 MPa
Tension: stress at stretching (long./perp.)* 23.5 /23.7 MPa
Tension: elongation at stretch (long./perp.)* 20.9 /26.4%
Tension: stress at rupture (long./perp.) 46.3 /45.0 MPa
Tension: elongation at rupture (long./perp.) 377 /356%
Tear resistance (500 mm/min) 37/37 N/mm (940 N/inch)
Point loading (500 gr. drop test) 350 mm
Stress/ elongation (50 mm/min.) 10 MPa/ 2 %
Physical-chemical properties
Melting range 265° C
Coefficient of thermal expansion 100 x 10-6 /° K
Transparencies of 0.1 mm foil (4 mills) 94%
UV-Transparency UV-transparent
Fire: classification B1 DIN 4102
Weathering, aging relatively fair (20-30 years)
Chemical resistance high resistance
*Initiation of stretching starts earlier!
Table 1 – Main properties of ETFE foils.
are done by compatible adhesive
tapes. Sound transmission from
inside to outside or inversely
should be studied carefully.
Membrane: ETFE membranes
are made of granular raw material
and extruded in a die of a size
of 1.55 m (status 2005). Regular
membranes are produced in a
thickness range from 0.08, 0.10,
0.15, and 0.2 to 0.25 mm. For air
cushions, a membrane thickness
of 0.2 to 0.25 mm (8 – 10 mills) is
Air cushions: The
1.55-m-wide (5.1 foot)
membranes are joined
by hot air and high frequency
welding techniques.
The cushion elements are
cut out of the rolled material by
CNC-controlled machines according
to their geometrical shape.
The borders of two or three membranes
are folded in loops and
then welded, and a flexible rod is
inserted in the loop. In a last step,
the rubber profiles are mounted
and then prepared for shipment.
On the construction site, the prefabricated
cushions are pulled
into a slotted metal profile so that
the loads are linearly transferred
to the supporting main structure.
A fault-free installation is
required to avoid stress concentrations.
Masoala Rainforest Building,
Zurich Zoo
The Masoala Rainforest
Building (MRF) at the Zurich Zoo
has a steel framework construction
covering an area of 120 x 90
meters (2.5 acres). Ten Virendeel
steel arches span 90 meters with
a flashing height of 31 meters.
Between the main arches, the roof
loads of the membrane structure
are supported by two lighter steel
trusses. Longitudinal loads are
transferred to the main structure
by interconnecting wind struts
(Figures 1, 2, and 3).
Flüeler – 32 Proceedings of the RCI 22nd International Convention
Figure 2 – Northwest view of the Masoala rain forest building of
the Zurich zoo.
Figure 1 – Tension elongation diagram of a 0.2-mm
ETFE membrane from a tensile test at room temperature.
Enlarged part: transition elastic to plastic behavior
Figure 3 – Mounting of the customized
ETFE air cushions with the help of
cable jacks.
The building envelope consists
of an inflatable triple membrane
made of ETFE foils jointed to loadcarrying
air cushions bridging the
4-m gap from the bearings to the
arc top. The border cushions span
the entire arc from bearing to
bearing with a total length of 100
meters. The air cushions are
formed by a wind-adaptive pressure
of 250 – 350 Pa (approx. 1 –
1.5% of a car tire pressure). Also,
the front sides are sealed by air
cushions.1 For maintenance and
cleaning, a mobile framework provides
access to the inside surfaces
of the air cushions. Lateral steel
flaps at the bearings and nine
large skylights in the roof provide
ventilation and support the climate
conditioning system.
Hailstorm 2002
The hailstorm of June 24,
2002, one week after the installation
and one year before the official
opening of the MRF, damaged
the exterior membranes of the
building envelope. All air cushions
of the roof except the front
sides had to be replaced. The
damage was caused mainly by the
size, shape, and intensity of that
hailstorm. Knobbed and strangely
shaped hailstones sized up to 70
mm caused the pressure drop of
the cushions and water intrusions
due to multiple perforations. Repair
of such heavy damage was
extremely time
consuming, costly,
and risky.
Development of
a Protection
Within a very
short time, a plan
to avoid future
hail damage had
to be established,
evaluated, and
tested in order to
guarantee the
opening date in
June 2003. In
order to maintain
insurance coverage,
a protection
system of the air
cushions was
mandatory. At
that time, no
membrane build-
Proceedings of the RCI 22nd International Convention Flüeler – 33
Figure 4 – Lateral bearings of the steel arches with three air
cushions and lateral air flaps for the ventilation. Distance
between the main beams, 12 meters.
Figure 5 – Typical hail size and shape of hailstorm June 2002. Swiss 2-franc
coin with diameter of 27 mm (about 1.1-inch diameter). Photo, U. Spreiter,
ing worldwide had such a protection. Also, experiences
about achieved protection effects of such systems
were missing. The following installations were
taken in consideration: for temporary or permanent
use, stiff (transparent plates) or flexible systems
(nets, foils, etc.).
After studying pros and cons, costs estimation,
and the available timeframe of only four months
(the planting of the forest was already underway),
the zoo decided to install a permanent protection
system in the form of an additional membrane.
The membrane of the same thickness covers the
air cushions and is inflated by a separate pressure
system. It is mounted to the framework, avoiding
interference with the sub-laid membrane structure.
Therefore, replacement is possible at any
The efficiency of the protection system was
tested in two steps: as single-layered and doublelayered
membranes with varying spacing pressurized
at 150 Pa. The hail impact was simulated by
polymer and ice balls of 40 and 50 mm with a
velocity range of 5 – 60 m/s. The displacement of
the membrane was measured by a high-speed
camera. Typical characteristics such as stretching
initiation, crack appearance, perforation of the top
and bottom membranes were recorded.
Flüeler – 34 Proceedings of the RCI 22nd International Convention
Figure 7 – Experimental arrangement for hail impact tests on the inflated ETFE
membrane, size 2.0 x 1.7 m2.
Figure 6 – Realized hail protection with an additional
ETFE layer, exchangeable without interference
to the main building envelope.
The optimal thickness of the
membrane was found by testing
with impacting polymer balls of Ø
40 mm. Figure 8 shows the kinetic
energy for various velocities in
relation to the membrane displacement.
At a velocity of 30 m/s
equal to 13.6 Joules, impacting
ice balls caused indents with
increasing stretching to the air
cushions. At 50 m/s, equal to 36
Joules, the top membrane perforated
and the bottom membrane
showed stretching phenomena.
However, impacting with polymer
balls caused similar phenomena
already at 20 m/s and respectively
at 52 m/s. The difference
results from friction, surface temperature,
and its heat transfer.2
AUGUST 7, 2004
Installation of the Protection
First, the perforated air cushion
of the building envelope had to
be replaced and re-pressurized.
Then, the protection membranes
equipped with the EPDM profiles
were installed. Due to the arch
shape of the trusses, the boundaries
of the protection membrane
had to be screwed to the existing
Alu-profile with a spacing of 15
cm (6 inches). Special attention
was paid to waterproofing and to
wind forces. The air pressure generators
were mounted on the top
of each main steel arch (12 systems
connecting three protection
Full-scale Hail-proof Test
On July 8, 2004, the region of
Aargau in Zurich was struck
again by a severe hailstorm.
Compared with 2002, the hailstones
were of smaller size, but
again, they were knobbed and
sharply edged, causing higher
impact energy per unit area
(Figure 11).
The protection membrane of
the MRF building was impacted
and perforated by numerous hits
– mainly on the exposed northwest
faces. It fully served its protection
purpose and no air cushions
were damaged. Until now,
the protection membrane has not
yet been replaced, for ecological
reasons. It will provide sufficient
protection during future hailstorms.
European regions such as the
Alps, the Pyrennes, the Apennine,
the Carpats, as well as other
areas, are frequently hit by hailstorms.
Due to the climate
changes, their frequency has increased.
Hailstones have changed
to more frequent irregular shape
and bigger size.3 Very often, the
hailstone size exceeds 40 mm (1.6
inch), the recommended size for
hail impact resistance in Switzerland.
Insurance companies are
aware of the new risks. They are
in the process of changing their
compensation policies accordingly
and pressing for stricter building
codes.4 The Swiss standards have
Proceedings of the RCI 22nd International Convention Flüeler – 35
Figure 8 – Kinetic energy vs. deflection of 40 mm PA-balls
for different membrane thicknesses.
Figure 9 – High-speed sequence of a Ø 40 mm PA sphere
impacting a single 200 μ ETFE-membrane. At a velocity
of 50.8 m/s, a total deflection of 108.5 mm was observed
causing small cracks after extended stretching. Background:
10 and 50 mm grid.
already implemented hail impact
as a regular load as well as protection
measures.5 Also, a classification
of hail impact resistance
for building materials will be
introduced soon. It will describe
the damage-free range for relevant
hailstone sizes.6 Engineers, architects,
and building owners are
well advised to evaluate the hail
impact risks already in the design
ETFEmembrane constructions
and similar thin building envelopes
are susceptible to impact
loads. The shape and nature of
the striking object play an especially
important role. At the MRF
building, it was demonstrated
that an efficient protection can be
installed without change in
appearance and major limitations
in use. In addition, limited protection
against flying, wind-driven
objects is also
achieved. Extended
test series
showed that
a spacing of
100 mm (4 in)
between the protection
and the air
cushion with a
minimal pressure
of 150 Pa
is sufficient. It
w i t h s t a n d s
hailstones up
to 50 mm withoutdamaging
bottom membrane.
Furthermore, the experiments
have shown that the hailstorm
conditions are simulated more
realistically by using ice balls.
During impact, the heat dissipates
faster with the use of ice
balls, due to a large contact surface
with the membrane, a high
stretching rate during an impact,
and the surface friction. In hail
impact experiments, it is therefore
highly recommended to use ice
balls of constant quality.
A special thanks goes to Irina
Makaorova, who contributed to
the investigation by her great testing
skills, and to A. Hohl, F. Oss,
and K. Kruppa, who accompanied
the activities with their high motivation.
The investigation received
financial support, which is highly
appreciated, from the Zurich Zoo,
the Cantonal Building Insurance
Co. of Zurich, and Covertex company.
Flüeler – 36 Proceedings of the RCI 22nd International Convention
Figures 10A and 10B – A: Installed protection membrane; B: EPDM rubber profile fastened to the
main structure.
Figure 11 – Typical hailstones of July 8, 2004,
collected by staff of the Zurich Zoo, partly melted
with sharp edges and diameters until 43 mm
1. Covertex, Obing, MFRbuilding,
technical data
2. P. Flüeler, I. Makarova,
EMPA Report No. 425’873,
Nov. 14, 2002 (not public).
3. S. Willemse, “A Statistical
Analysis and Climatological
Interpretation of Hailstorms
in Switzerland,”
PhD Thesis ETH, No. 1137,
4. P. Zimmerli, “Hailstorms in
Europe, New View to a
Known Risk,” Focus report,
Swiss Re, Zürich, No.
1501360, May 2005.
5. Sia, 261/1: Einwirkung auf
Tragwerke, Hagel, standard
of Swiss Society of Engineers
and Architects, 2001.
6. W. Ernst: Dachabdichtung
(waterproofing of roofs),
Dachbegrünung (green
roof), Probleme (problems);
chapt.1.2: Hail (P. Flüeler),
ISBN 3-00-017011-1,1.
edition Aug. 2005.
Proceedings of the RCI 22nd International Convention Flüeler – 37
Figure 12 – Protection membrane after hailstorm of July 8, 2004. It suffered numerous
perforations and indents but air cushions were undamaged.